Even though there have been significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties when employing them to provide a high quality image display. Particularly, as the display system of the HDTV format becomes popular, an image size on a screen becomes increasingly bigger such as 100″ or more in the diagonal size. The pixel size on the screen is more than 1 mm when specification is 100″-size image including 1920×1080 pixels. Similarly, in a 50″-size image and XGA pixels, the pixel size is 1 mm. An observer can see each of the pixels on the screen, for these reasons, the display systems require a high number of gray scales of more than 10-bit or 16-bit, in order to provide a high quality display system. Furthermore, when the display image is digitally controlled, the image quality is adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interests because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of required devices ranges from 60,000 to several millions for each SLM. Referring to FIG. 1A for a digital video system 1 disclosed in a reference U.S. Pat. No. 5,214,420 that includes a display screen 2. A light source 10 is used to generate light energy for an ultimate illumination of display screen 2. The generated light 9 is further concentrated and directed toward lens 12 by way of a mirror 11. Lenses 12, 13 and 14 form a beam columnator operating to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over a data cable 18 to selectively redirect a portion of the light from a path 7 toward a lens 5 for displaying in a screen 2. The SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30 that is shown in FIG. 1B. When element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along a path 4 to impinge the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, the light is not redirected toward display screen 2 and hence the pixel 3 is dark.
The on- and off-states of a micromirror control scheme as implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display system impose a limitation on the quality of the display. Specifically, an application of a conventional configuration of control circuit is faced with a limitation that the gray scale of conventional system (PWM between ON and OFF states) is limited by an LSB (i.e., least significant bit, or the least pulse width). Due to the ON-OFF states implemented in the conventional systems, there is no way to provide a shorter pulse width than the LSB. The least brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, FIG. 1C shows an exemplary circuit diagram of control circuit of a prior art for a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes a memory cell 32. Various transistors are referred to as “M*” where designates a transistor number, and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; and transistors M6, M8, and M9 are n-channel transistors. The capacitances C1 and C2 represent the capacitive loads presented to the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of a static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a wordline. Latch 32a is formed from two cross-coupled inverters, i.e., M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The dual-state switching as illustrated by the control circuit controls the micromirrors to position either at an ON or OFF angular orientation as shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally control image system, is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8 that in turn define the relative brightness of each of the four bits where the 1 is for the least significant bit and the 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable difference between gray scales for displaying images at different levels of light intensities is a quantity of light intensity represented by a “least significant bit” that maintains the micromirror at an ON position for a shortest controllable duration.
When adjacent image pixels are shown with a great degree of different gray scales due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are specially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It has been observed in an image of a female model that there are artifacts shown on the forehead, the sides of the nose and the upper arm. The artifacts are generated due to a technical limitation that the digitally controlled display does not provide sufficient gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have either a fully on or a fully off positions, the light intensity is determined by the length of time the micromirror is at the fully on position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased such that the digital control signals can be increased to a higher number of bits.
However, when the speed of the micromirrors is increased, a stronger hinge is necessary for the micromirror to sustain a required number of operational cycles for a designated lifetime of operation. In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The micromirrors manufactured by applying the CMOS technologies probably would not be suitable for operation at such a high range of voltages and therefore DMOS micromirror devices may be required. In order to achieve higher degree of gray scale control, a more complicated manufacturing process and larger device areas are necessary when the DMOS micromirror is implemented. Conventional modes of micromirror control are therefore faced with a technical challenge that the gray scale accuracy has to be sacrificed for the benefits of smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to a light intensity control. These patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different shapes of light sources. These patents includes U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. The U.S. Pat. No. 6,746,123 has disclosed special polarized light sources for preventing light loss. However, these patents or patent application does not provide an effective solution to overcome the limitations caused by insufficient gray scales in the digitally controlled image display systems.
Furthermore, there are many patents related to a spatial light modulation that includes U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,615,595, 4,728,185, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,506,597, 5,489,952, 6064,366, 6535,319, and 6,880,936. However, these inventions do not address or provide a direct resolution for a person of ordinary skills in the art to overcome the above-discussed limitations and difficulties.
Therefore, a need still exists in the art of image display systems applying digital controls of a micromirror array as a spatial light modulator to provide a new and improved system such that the above-discussed difficulties can be resolved.